If you need to make credits quick, you can use 2 methods: sell Cartel Market items, or buy low, sell high. For the Cartel Market, you want to buy items worth a lot on the GTN. For the buy low, sell high method, you just buy items on the GTN for cheap, then sell them for a higher price. You have to watch prices though, and make sure you know it will sell for the price you are selling it for. You can also just go on YouTube and look up videos about making credits.